494 Mr. William Croohes [June 11, 



and under the stress of this pressure the dissolved carbon separates 

 out in a transparent, dense, crystalline form — in fact, as diamond. 



Now commences the tedious part of the process. The metallic 

 inf^ot is attacked with hot nitro-hjdrochloric acid until no more iron 

 is dissolved. The bulky residue consists chiefly of graj^hite, together 

 with translucent flakes of a chestnut-coloured carbon, black opaque 

 carbon of a density of from 3 * to 3*5, and hard as diamonds — black 

 diamonds or carbonado, in fact — and a small portion of transparent 

 colourless diamonds showing crystalline structure. Besides these, 

 there may be carbide of silicon and corundum, arising from impurities 

 in the materials employed. 



The residue is first heated for some hours with strong sulphuric 

 acid at the boiling point, with the cautious addition of powdered nitre. 

 It is then well washed and allowed for two days to soak in strong 

 hydrofluoric acid in the cold, then in boiling acid. After this treat- 

 ment the soft graphite will disappear, and most, if not all, of the 

 silicon compounds will be destroyed. Hot sulphuric acid is again 

 applied to destroy the fluorides, and the residue, well washed, is 

 repeatedly attacked with a mixture of the strongest nitric acid and 

 powdered potassium chlorate, kept warm, but to avoid explosions not 

 above 60° C. This ceremony must be repeated six or eight times, 

 when all the hard graphite will gradually be dissolved, and little 

 else left but graphitic oxide, diamond and the harder carbonado and 

 boart. The residue is fused for an hour in fluorhydrate of fluoride 

 of potassium, then boiled out in water, and again heated in sulphuric 

 acid. The well-washed grains which resist this energetic treatment 

 are dried, carefully deposited on a slide, and examined under the 

 microscope. Along with numerous pieces of black diamond are seen 

 transparent colourless pieces, some amorphous, others with a crystal- 

 line appearance, as I have attempted to reproduce in drawings. 

 Although many fragments of crystals occur, it is remarkable that I 

 have never seen a complete crystal. All appear broken up, as if on 

 being liberated from the intense pressure under which they were 

 formed they burst asunder. I have direct evidence of this phe- 

 nomenon. A very fine piece of artificial diamond, carefully mounted 

 by me on a microscopic slide, exploded during the night and covered 

 my slide with fragments. This bursting paroxysm is not unknown 

 at the Kimberley mines. 



On the screen I will project fragments of artificial diamond 

 (Figs. 26, 27), some lent me by Professor Roberts- Austen, others 

 of my own make ; while on the wall you will see drawings of dia- 

 monds copied from M. Moissan's book on the Electric Furnace. Un- 

 fortunately these specimens are all microscopic. The largest arti- 

 ficial diamond, so far, is less than one millimetre across. 



Laboratory diamonds burn in the air before the blowpipe to 

 carbonic acid ; and in lustre, crystalline form, optical properties, 

 density and hardness they are identical with the natural stone. 



Many circumstances point to the conclusion that the diamond 



